Over the Holidays I had a chance to measure current in loading coils. The
data is extensive and will take time to analyze fully, but I already put a
table of some measurements up on my site at:
http://www.w8ji.com/mobile_and_loaded_antenna.htm
The short of it is this:
Current has nothing to do with the fact it is a "loading coil" as opposed to
a regular inductor.
In a toroid, the current is so close to equal at both ends I cannot resolve
any difference from the input lead to output. Using an air-inductor, the
current change ranges from very low (4%) in a compact inductor to
significant (27%) in a relatively large longer (one foot) inductor.
The current change is proportional to the distributed capacitance between
the inductor and "earth" and the load impedance presented by whatever is
beyond that point. The bulk of current taper, if there is any, is in the
upper half of the coil.
Over the next few months I'll get a full analysis up on my website and post
it, but for now it looks like conventional theory applies without any
unforeseen problems.
73, Tom W8JI
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